Check crankshaft seals for leakage at both ends of the engine, look for leaks around the oil pan gasket and all lubrication system connections. Check to see if oil is being blown from the crankcase breather. This can be caused by combustion gases leaking past pistons. Clogged crankcase breathers will contribute to high crankcase pressure and result in gasket and seal leaks.
On turbocharged engines, leaking turbocharger shaft seals (turbine end) will cause loss of oil through the exhaust gases.
Internal Leakage Into Upper Cylinders
Upper cylinder oil leakage can be the cause of blue smoke. There are four possible routes for oil leakage into upper cylinders:
Oil can leak past the ring seals at the impeller end of the turbocharger shaft.
Leakage between worn guides and valves is also possible.
Worn or damaged piston rings or plugged oil return holes can cause oil to enter the upper cylinder. Incorrectly installed compression rings will cause oil to pump into the cylinders.
Excess oil usage can also be the result of oil viscosity being too low for prevailing conditions. Low oil viscosity can be caused by crankcase dilution, incorrect selection of oil weights, or engine overheating.